High temperature oxidation protection for composites

ABSTRACT

Systems and methods for forming an oxidation protection system on a composite structure are provided. In various embodiments, an oxidation protection system disposed on a substrate may comprise a boron-silicon-glass layer formed directly on the composite structure. The boron-silicon-glass layer may comprise a boron compound, a silicon compound, and a glass compound.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of, and claims priority to and thebenefit of, U.S. application Ser. No. 16/590,170, filed Oct. 1, 2019,and titled “HIGH TEMPERATURE OXIDATION PROTECTION FOR COMPOSITES,” whichis incorporated herein by reference in its entirety for all purposes.

FIELD

The present disclosure relates generally to composites and, morespecifically, to oxidation protection systems for carbon-carboncomposite structures.

BACKGROUND

Oxidation protection systems for carbon-carbon composites are typicallydesigned to minimize loss of carbon material due to oxidation atoperating conditions, which include temperatures of 900° C. (1652° F.)or higher. Phosphate-based oxidation protection systems may reduceinfiltration of oxygen and oxidation catalysts into the compositestructure. However, despite the use of such oxidation protectionsystems, significant oxidation of the carbon-carbon composites may stilloccur during operation of components such as, for example, aircraftbraking systems. In addition, at such high operating temperatures,phosphate-based oxidation protection systems (OPS) applied to non-wearsurfaces of brake disks may experience decreasing viscosity, which maycause the OPS to migrate away from non-wear surface edges proximate to awear surface of the brake disk, leaving the composite material at orproximate to the non-wear surface edges vulnerable to oxidation.Oxidation protection system having coatings of boron carbide and siliconcarbide applied via chemical vapor deposition (CVD) have demonstratedeffective oxidation protection at high operating temperature. However,the CVD process increases manufacturing costs.

SUMMARY

A method for forming an oxidation protection system, on a compositestructure is provided. In accordance with various embodiments, themethod may comprise applying a boron-silicon-glass composite slurry tothe composite structure, and heating the composite structure to atemperature sufficient to form a boron-silicon-glass layer on thecomposite structure. The boron-silicon-glass composite slurry maycomprise a boron compound, a silicon compound, a glass compound, and acarrier fluid.

In various embodiments, the boron compound may comprise a boron carbideand the silicon compound may comprise silicon carbide. In variousembodiments, the glass compound may comprise a borosilicate glass.

In various embodiments, the boron carbide may comprise a first group ofboron carbide particles having a first average particle size and asecond group of boron carbide particles having a second average particlesize greater than the first average particle size. In variousembodiments, the second group of boron carbide particles may form agreater weight percentage of the boron-silicon-glass composite slurrythan the first group of boron carbide particles.

In various embodiments, an average particle size of a first group ofsilicon carbide particles of the silicon carbide may be less than anaverage particle size of a second group of silicon carbide particles ofthe silicon carbide. In various embodiments, the first group of siliconcarbide particles may form a weight percentage of theboron-silicon-glass composite slurry that is greater than a weightpercentage of the boron-silicon-glass composite slurry formed by thesecond group of silicon carbide particles.

In various embodiments, the boron compound may comprise at least one oftitanium diboride, boron nitride, boron carbide, zirconium boride,silicon hexaboride, or elemental boron, and the silicon compound maycomprise at least one of silicon carbide, silicon dioxide, a silicidecompound, silicon, fumed silica, or silicon carbonitride. In variousembodiments, the silicon compound may comprise silicon carbide particleshaving an average particle size of less than or equal to 1 micrometer.In various embodiments, the boron compound may comprise boron carbideparticles having an average particle size of greater than or equal to9.0 micrometers.

In various embodiments, heating the composite structure to thetemperature sufficient to form the boron-silicon-glass layer on thecomposite structure may comprise heating the composite structure at afirst temperature of about 300° Fahrenheit for between 10 minutes and1.5 hours, and heating the composite structure at a second temperatureof about 1650° Fahrenheit for between 1.5 hours and 2.5 hours.

In various embodiments, applying the boron-silicon-glass compositeslurry to the composite structure comprises at least one of brushing orspraying. The boron-silicon-glass composite slurry may be applieddirectly on a surface of the composite structure.

An oxidation protection system disposed on an outer surface of asubstrate is also disclosed herein. In accordance with variousembodiments, the oxidation protection system may comprise aboron-silicon-glass layer disposed on the outer surface. Theboron-silicon-glass layer may comprise a boron compound, a siliconcompound, and a glass compound.

In various embodiments, the boron compound may comprise a first group ofboron particles each having a first average particle size and a secondgroup of boron particles each having a second average particle sizegreater than the first average particle size. In various embodiments, anaverage particle size of a first group of silicon carbide particles ofthe silicon compound may be less than an average particle size of asecond group of silicon carbide particles of the silicon compound.

In various embodiments, the boron-silicon-glass layer may be formed indirect contact with the outer surface of the substrate. In variousembodiments, the glass compound may comprise a borosilicate glass.

In various embodiments, the boron compound may comprise at least one oftitanium diboride, boron nitride, boron carbide, zirconium boride,silicon hexaboride, or elemental boron, and the silicon compound maycomprise at least one of silicon carbide, silicon dioxide, a silicidecompound, silicon, fumed silica, or silicon carbonitride.

In accordance with various embodiments, a method for forming anoxidation protection system on a composite structure may comprise applya boron slurry to the composite structure, performing a first lowtemperature bake by heating the composite structure at a firsttemperature, applying a silicon slurry to the composite structure,performing a second low temperature bake by heating the compositestructure at a second temperature, and performing a high temperatureheat treatment by heating the composite structure at a thirdtemperature. The third temperature may be greater than the firsttemperature and the second temperature.

In various embodiments, the first temperature may be about 300°Fahrenheit, the second temperature may be about 300° Fahrenheit, and thethird temperature may be about 1650° Fahrenheit.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the present disclosure is particularly pointed outand distinctly claimed in the concluding portion of the specification. Amore complete understanding of the present disclosure, however, may bestbe obtained by referring to the detailed description and claims whenconsidered in connection with the drawing figures, wherein like numeralsdenote like elements.

FIG. 1A illustrates a cross sectional view of an aircraft wheel brakingassembly, in accordance with various embodiments;

FIG. 1B illustrates a partial side view of an aircraft wheel brakingassembly, in accordance with various embodiments;

FIG. 2 illustrates a method for forming an oxidation protection systemon a composite structure, in accordance with various embodiments;

FIG. 3 illustrates experimental data obtained from testing variousoxidation protection systems, in accordance with various embodiments;and

FIG. 4 illustrates a method for forming an oxidation protection systemon a composite structure, in accordance with various embodiments.

DETAILED DESCRIPTION

The detailed description of embodiments herein makes reference to theaccompanying drawings, which show embodiments by way of illustration.While these embodiments are described in sufficient detail to enablethose skilled in the art to practice the disclosure, it should beunderstood that other embodiments may be realized and that logical andmechanical changes may be made without departing from the spirit andscope of the disclosure. Thus, the detailed description herein ispresented for purposes of illustration only and not for limitation. Forexample, any reference to singular includes plural embodiments, and anyreference to more than one component or step may include a singularembodiment or step. Also, any reference to attached, fixed, connected orthe like may include permanent, removable, temporary, partial, fulland/or any other possible attachment option.

With initial reference to FIGS. 1A and 1B, aircraft wheel brakingassembly 10 such as may be found on an aircraft, in accordance withvarious embodiments is illustrated. Aircraft wheel braking assembly may,for example, comprise a bogie axle 12, a wheel 14 including a hub 16 anda wheel well 18, a web 20, a torque take-out assembly 22, one or moretorque bars 24, a wheel rotational axis 26, a wheel well recess 28, anactuator 30, multiple brake rotors 32, multiple brake stators 34, apressure plate 36, an end plate 38, a heat shield 40, multiple heatshield sections 42, multiple heat shield carriers 44, an air gap 46,multiple torque bar bolts 48, a torque bar pin 50, a wheel web hole 52,multiple heat shield fasteners 53, multiple rotor lugs 54, and multiplestator slots 56. FIG. 1B illustrates a portion of aircraft wheel brakingassembly 10 as viewed into wheel well 18 and wheel well recess 28.

In various embodiments, the various components of aircraft wheel brakingassembly 10 may be subjected to the application of compositions andmethods for protecting the components from oxidation.

Brake disks (e.g., interleaved rotors 32 and stators 34) are disposed inwheel well recess 28 of wheel well 18. Rotors 32 are secured to torquebars 24 for rotation with wheel 14, while stators 34 are engaged withtorque take-out assembly 22. At least one actuator 30 is operable tocompress interleaved rotors 32 and stators 34 for stopping the aircraft.In this example, actuator 30 is shown as a hydraulically actuatedpiston, but many types of actuators are suitable, such as anelectromechanical actuator. Pressure plate 36 and end plate 38 aredisposed at opposite ends of the interleaved rotors 32 and stators 34.Rotors 32 and stators 34 can comprise any material suitable for frictiondisks, including ceramics or carbon materials, such as a carbon/carboncomposite.

Through compression of interleaved rotors 32 and stators 34 betweenpressure plates 36 and end plate 38, the resulting frictional contactslows rotation of wheel 14. Torque take-out assembly 22 is secured to astationary portion of the landing gear truck such as a bogie beam orother landing gear strut, such that torque take-out assembly 22 andstators 34 are prevented from rotating during braking of the aircraft.

Carbon-carbon composites (also referred to herein as compositestructures, composite substrates, and carbon-carbon compositestructures, interchangeably) in the friction disks may operate as a heatsink to absorb large amounts of kinetic energy converted to heat duringslowing of the aircraft. Heat shield 40 may reflect thermal energy awayfrom wheel well 18 and back toward rotors 32 and stators 34. Withreference to FIG. 1A, a portion of wheel well 18 and torque bar 24 isremoved to better illustrate heat shield 40 and heat shield segments 42.With reference to FIG. 1B, heat shield 40 is attached to wheel 14 and isconcentric with wheel well 18. Individual heat shield sections 42 may besecured in place between wheel well 18 and rotors 32 by respective heatshield carriers 44 fixed to wheel well 18. Air gap 46 is definedannularly between heat shield segments 42 and wheel well 18.

Torque bars 24 and heat shield carriers 44 can be secured to wheel 14using bolts or other fasteners. Torque bar bolts 48 can extend through ahole formed in a flange or other mounting surface on wheel 14. Eachtorque bar 24 can optionally include at least one torque bar pin 50 atan end opposite torque bar bolts 48, such that torque bar pin 50 can bereceived through wheel web hole 52 in web 20. Heat shield sections 42and respective heat shield carriers 44 can then be fastened to wheelwell 18 by heat shield fasteners 53.

Under the operating conditions (e.g., high temperature) of aircraftwheel braking assembly 10, carbon-carbon composites may be prone tomaterial loss from oxidation of the carbon. For example, variouscarbon-carbon composite components of aircraft wheel braking assembly 10may experience both catalytic oxidation and inherent thermal oxidationcaused by heating the composite during operation. In variousembodiments, composite rotors 32 and stators 34 may be heated tosufficiently high temperatures that may oxidize the carbon surfacesexposed to air. At elevated temperatures, infiltration of air andcontaminants may cause internal oxidation and weakening, especially inand around brake rotor lugs 54 or stator slots 56 securing the frictiondisks to the respective torque bar 24 and torque take-out assembly 22.Because carbon-carbon composite components of aircraft wheel brakingassembly 10 may retain heat for a substantial time period after slowingthe aircraft, oxygen from the ambient atmosphere may react with thecarbon matrix and/or carbon fibers to accelerate material loss. Further,damage to brake components may be caused by the oxidation enlargement ofcracks around fibers or enlargement of cracks in a reaction-formedporous barrier coating (e.g., a silicon-based barrier coating) appliedto the carbon-carbon composite.

Elements identified in severely oxidized regions of carbon-carboncomposite brake components include potassium (K) and sodium (Na). Thesealkali contaminants may come into contact with aircraft brakes as partof cleaning or de-icing materials. Other sources include salt depositsleft from seawater or sea spray. These and other contaminants (e.g., Ca,Fe, etc.) can penetrate and leave deposits in pores of carbon-carboncomposite aircraft brakes, including the substrate and anyreaction-formed porous barrier coating. When such contamination occurs,the rate of carbon loss by oxidation can be increased by one to twoorders of magnitude.

In various embodiments, brake disks of aircraft wheel braking assembly10 may reach operating temperatures in the range from about 100° C.(212° F.) up to about 900° C. (1652° F.), or higher (e.g., 1093° C.(2000° F.). However, it will be recognized that the oxidation protectionsystems compositions and methods of the present disclosure may bereadily adapted to many parts in this and other braking assemblies, aswell as to other carbon-carbon composite structures susceptible tooxidation losses from infiltration of atmospheric oxygen and/orcatalytic contaminants.

In various embodiments, a method for limiting an oxidation reaction in asubstrate (e.g., a composite structure) may comprise forming anoxidation protection system on the composite structure. With referenceto FIG. 2A, a method 200 for forming an oxidation protection system oncomposite structure is illustrated. In accordance with variousembodiments, method 200 may, for example, comprise applying an oxidationinhibiting composition to non-wearing surfaces of carbon-carboncomposite brake components, such as non-wear surfaces 45 and/or lugs 54.Non-wear surface 45, as labeled in FIG. 1A, simply references anexemplary non-wear surface on a brake disk, but non-wear surfacessimilar to non-wear surface 45 may be present on any brake disks (e.g.,rotors 32, stators 34, pressure plate 36, end plate 38, or the like). Invarious embodiments, method 200 may be used on the back face of pressureplate 36 and/or end plate 38, an inner diameter (ID) surface of stators34 including slots 56, as well as outer diameter (OD) surfaces of rotors32 including lugs 54. The oxidation inhibiting composition of method 200may be applied to preselected regions of a carbon-carbon compositestructure that may be otherwise susceptible to oxidation. For example,aircraft brake disks may have the oxidation inhibiting compositionapplied on or proximate stator slots 56, rotor lugs 54, and/or non-wearsurface 45. Method 200 may be performed on densified a carbon-carboncomposite. In this regard, method 200 may be performed aftercarbonization and densification of the carbon-carbon composite.

In various embodiments, method 200 may comprise forming aboron-silicon-glass composite slurry (step 210) by combining a boroncompound, a silicon compound, and a glass compound with a carrier fluid(such as, for example, water). In various embodiments, the boroncompound may comprise at least one boron-comprising refractory material(e.g., ceramic materials). In various embodiments, the boron compoundmay comprise titanium diboride, boron nitride, boron carbide, zirconiumboride, silicon hexaboride, and/or elemental boron.

The weight percentage of the boron compound within theboron-silicon-glass composite slurry may be any suitable weightpercentage for the desired application. In various embodiments, theboron-silicon-glass composite slurry may comprise from 5% to 50% byweight boron compound, from 10% to 40% by weight boron compound, from20% to 30% by weight boron compound, from 20% to 25% by weight boroncompound, and/or about 21.2% by weight boron compound, about 22.5% byweight boron compound, and/or about 24.1% by weight boron compound. Asused in this context only, the term “about” means plus or minus 1 weightpercent.

In various embodiments, the boron compound may comprise boron powder(e.g., boron carbide powder). The boron compound comprises particleshaving an average particle size between 100 nanometers (nm) and 100micrometers (μm) (between 3.9×10⁻⁶ inch and 0.0039 inch), between 500 nmand 100 μm (between 2×10⁻⁵ inch and 0.0039 inch), between 500 nm and 1μm (between 2×10⁻⁵ inch and 3.9×10⁻⁵ inch), between 1 μm and 50 μm(between 3.9×10⁻⁵ inch and 0.002 inch), between 1 μm and 20 μm (between3.9×10⁻⁵ inch and 0.0008 inch), and/or between 1 μm and 10 μm (between3.9×10⁻⁵ inch and 0.0004 inch), about 0.7 μm (2.8×10⁻⁵ inch), about 9.3μm (0.0004 inch), and/or about 50 μm (0.0020 inches). As used in thiscontext only, the term “about” means plus or minus ten percent of theassociated value.

In various embodiments, the boron compound may comprise particles ofvarying size. In various embodiments, the boron compound comprises afirst group of particles having a first average particle size and asecond group of particles having a second average particle size greaterthan the first average particle size.

In various embodiments, the particles of the first group may have anaverage particle size between 100 nm and 20 μm (between 3.9×10⁻⁶ inchand 0.0008 inch), between 500 nm and 10 μm (between 2×10⁻⁵ inch and0.0004 inch), between 500 nm and 1.0 μm (2×10⁻⁵ inch and 3.9×10⁻⁵ inch),and/or about 0.7 μm (2.8×10⁻⁵ inch). As used in this context only, theterm “about” means plus or minus ten percent of the associated value.

In various embodiments, the particles of the second group may have anaverage particle size between 500 nm and 60 μm (2×10⁻⁵ inch and 0.0008inch), 1 μm and 15 μm (between 2×10⁻⁵ and 0.0006 inch), between 8 μm and10.0 μm (between 0.0003 inch and 0.0008 inch), between 45 μm and 55 μm(between 0.0018 inch and 0.0022 inch), about 50 μm (0.0020 inch), and/orabout 9.3 μm (0.00039 inch). As used in this context only, the term“about” means plus or minus ten percent of the associated value.

In various embodiments, the boron compound particles of the secondaverage particle size form a larger weight percentage of theboron-silicon-glass composite slurry as compared to the weightpercentage formed by the boron compound particles of the first averagesize. In various embodiments, the boron-silicon-glass composite slurrymay comprise from 0% to 20% by weight the boron compound first averageparticle size, 5% to 8% by weight the boron compound first averageparticle size, about 6.36% by weight the first boron compound averageparticle size, and/or about 7.24% by weight the first boron compoundaverage particle size. As used in this context only, the term “about”means plus or minus 0.5% weight percent. In various embodiments, theboron-silicon-glass composite slurry may comprise from 5% to 30% byweight the second boron compound average particle size, 10% to 25% byweight the second boron compound average particle size, 14% to 23% byweight the second boron compound average particle size, about 16.9% byweight the second boron compound average particle size, about 21.2% byweight the second boron compound average particle size, and/or about22.6% by weight the second boron compound average particle size. As usedin this context only, the term “about” means plus or minus 0.5% weightpercent.

In various embodiments, the silicon compound may comprise siliconcarbide, a silicide compound, silicon, or silicon carbonitride. Theweight percentage of the silicon compound within the boron-silicon-glasscomposite slurry may be any suitable weight percentage for the desiredapplication. In various embodiments, the boron-silicon-glass compositeslurry may comprise from 10% to 40% by weight silicon compound, from 20%to 35% by weight silicon compound, from 27% to 32% by weight siliconcompound, about 27.3% by weight silicon compound, about 31% by weightsilicon compound, and/or about 31% by weight silicon compound. As usedin this context only, the term “about” means plus or minus 2 weightpercent.

In various embodiments, the silicon compound may comprise a siliconpowder (e.g., silicon carbide powder). The silicon compound comprisesparticles, the silicon compound average particle size may be between 100nm and 50 μm (between 3.9×10⁻⁶ inch and 0.0039 inch), between 500 nm and20 μm (between 2×10⁻⁵ inch and 0.0039 inch), between 500 nm and 1.5 μm(between 2×10⁻⁵ inch and 3.9×10⁻⁵ inch), between 15 μm and 20 μm(between 3.9×10⁻⁵ inch and 0.002 inch), about 17 μm (0.0007 inch),and/or about 1.0 μm (4×10⁻⁵ inch). As used in this context only, theterm “about” means plus or minus ten percent of the associated value.

In various embodiments, the silicon compound may comprise siliconcompound particles of varying size. In various embodiments, the siliconcompound comprises a first group of silicon compound particles having afirst average particle size and a second group of silicon compoundparticles having a second average particle size. The first siliconcompound average particle size may be less than the second siliconcompound average particle size.

In various embodiments, the silicon compound particles of the firstgroup have an average particle size between 100 nm and 20 μm (between3.9×10⁻⁶ inch and 0.0008 inch), 500 nm and 10 μm (between 2×10⁻⁵ and0.0004 inch); between 500 nm and 1.5 μm (between 2×10⁻⁵ and 6×10⁻⁵),and/or about 1.0 μm (4×10⁻⁵ inch). In various embodiments, the siliconcompound particles of the second group may have an average particle sizebetween 1 μm and 50 μm (between 4×10⁻⁵ inch and 0.002 inch), between 5μm and 25 μm (between 0.0002 inch and 0.001), between 16 μm and 18 μm(between 0.0006 inch and 0.0007 inch), and/or about 17 μm (0.00067inches). As used in this context only, the term “about” means plus orminus ten percent of the associated value.

In various embodiments, the silicon compound particles of the firstaverage particle size form a larger weight percentage of theboron-silicon-glass composite slurry as compared to the weightpercentage formed by the silicon compound particles of the secondaverage particle size. In various embodiments, the boron-silicon-glasscomposite slurry may comprise from 0% to 30% by weight the secondsilicon compound average particle size, 0% to 25% by weight the secondsilicon compound average particle size, about 9.3% by weight the secondsilicon compound average particle size, and/or about 21.7% by weight thesecond silicon compound average particle size. As used in this contextonly, the term “about” means plus or minus 1% weight percent. In variousembodiments, the boron-silicon-glass composite slurry may comprise from5% to 40% by weight the first silicon compound average particle size,10% to 35% by weight the first silicon compound average particle size,20% to 30% by weight the first silicon compound average particle size,about 21.7% by weight the second silicon compound average particle size,about 27.2% by weight the second silicon compound average particle size,and/or about 29.0% by weight the second silicon compound averageparticle size. As used in this context only, the term “about” means plusor minus 1% weight percent.

In various embodiments, the glass compound is a borosilicate glasscomposition in the form of a glass frit, powder, or other suitablepulverized form. In various embodiments, the borosilicate glasscomposition may comprise silicon dioxide (SiO₂), boron trioxide (B₂O₃),and/or aluminum oxide (Al₂O₃). The borosilicate glass composition maycomprise in weight percentage of 13% B₂O₃, 61% SiO₂, 2% Al₂O₃, and 4%Na₂O, and may have a coefficient of thermal expansion (CTE) of 3.3×10⁻⁶cm/Cm, a working point of 2286° F. (1252° C.), and an annealing point of1040° F. (560° C.). In various embodiments, the glass compound maycomprise a lithia potash borosilicate glass composition in the form of aglass frit, powder, or other suitable pulverized form and having a CTEof 3.0×10⁻⁶ cm/Cm, a working point of 1954° F. (1068° C.), and anannealing temperature of 925° F. (496° C.). In various embodiments, theglass compound may comprise borophosphates, a borosilicate compositionincluding in weight percentage, 96% SiO₂ and 4% B₂O₃ (which is availableunder the trade name VYCOR® from Corning Incorporated of Corning, N.Y.,USA), quartz, aluminosilicate, boroaluminosilicate, and/or any othersuitable glass compound, which may be in the form of a glass frit,powder, or other suitable pulverized form.

The weight percentage of the glass compound within theboron-silicon-glass composite slurry may be any suitable weightpercentage for the desired application. In various embodiments, theboron-silicon-glass composite slurry may comprise from 1% to 35% byweight glass compound, from 5% to 25% by weight glass compound, from 5%to 8% by weight glass compound, from 5% to 8% by weight glass compound,from 11% to 13% by weight glass compound, from 17.5% to 19% by weightglass compound, about 6.9% by weight glass compound, about 12.9% byweight glass compound, and/or about 18.2% by weight glass compound. Asused in this context only, the term “about” means plus or minus 0.5weight percent.

In various embodiments, the glass compound comprises particles, theaverage particle size may be between 500 nm and 50 μm (between 3.9×10⁻⁶inch and 0.0039 inch), between 5 μm and 25 μm (between 2×10⁻⁵ inch and0.0039 inch), between 10 μm and 15 μm (between 2×10⁻⁵ inch and 3.9×10⁻⁵inch), between 11.5 μm and 13 μm (between 3.9×10⁻⁵ inch and 0.002 inch),and/or between about 12.3 μm (0.0004 inch). As used in this contextonly, the term “about” means plus or minus 0.5 weight percent.

The remaining weight percent of the boron-silicon-glass composite slurryother than the boron compound, the silicon compound, and the glass maycomprise the carrier fluid and/or any other suitable additives. Invarious embodiments, the boron-silicon-glass composite slurry consistsof boron carbide, silicon carbide, borosilicate glass, and water. Invarious embodiments, the boron-silicon-glass composite slurry may besubstantially free of phosphate. In this case, “substantially free”means less than 0.01 percent by weight of the boron-silicon-glasscomposite slurry.

In various embodiments, the boron slurry may comprise about 7.241% byweight boron carbide having an average particle size of about 0.7 μm(2.8×10⁻⁵ inch), about 16.897% by weight boron carbide having an averageparticle size of about 9.3 μm (0.0004 inch), about 9.310% by weightsilicon carbide having an average particle size of about 17 μm (0.0007inch), about 21.724% by weight silicon carbide having an averageparticle size of about 1 μm (4×10⁻⁵ inch), about 6.897% by weightborosilicate glass having an average particle size of about 12.3 μm(0.0005 inch), and about 37.931% by weight water; the borosilicate glassbeing comprised, in weight percentage of the borosilicate glass, ofabout 13% B₂O₃, about 61% SiO₂, about 2% Al₂O₃, and about 4% Na₂O. Asused in this context only, the term “about” plus or minus ten percent ofthe associated value.

In various embodiments, method 200 further comprises applying theboron-silicon-glass composite slurry to a composite structure (step220). Applying the boron-silicon-glass composite slurry may comprise,for example, spraying or brushing the boron-silicon-glass compositeslurry to an outer surface of the composite structure. Embodiments inwhich the carrier fluid for the boron-silicon-glass composite slurry iswater causes the aqueous boron-silicon-glass composite slurry to be moresuitable for spraying or brushing application processes. In variousembodiments, the application of the boron-silicon-glass composite slurryto the composite structure may not comprise chemical vapor deposition(CVD), thus saving the significant monetary expense associated with CVD.Any suitable manner of applying the boron-silicon-glass composite slurryto the composite structure is within the scope of the presentdisclosure, except in various embodiments, CVD. As referenced herein,the composite structure may refer to a carbon-carbon compositestructure. In accordance with various embodiments, theboron-silicon-glass composite slurry may be applied directly on (i.e.,in physical contact with) the surface of the composite structure. Inthis regard, method 200 generally does not include a pretreatingcomposition and/or a pretreating step and/or forming a sealing layerprior to applying the boron-silicon-glass composite slurry, as may beassociated with other oxidation protection systems, for example,oxidation protection systems comprising phosphate glass layers.

In various embodiments, method 200 may further comprise a step 230 ofheating the composite structure to form a boron-silicon-glass layer onthe composite structure. In various embodiments, the boron-silicon-glasslayer may be formed directly adjacent to the composite structure. Theheating of the composite structure may remove the carrier fluid from theboron-silicon-glass composite slurry to form the boron-silicon-glasslayer.

In various embodiments, step 230 may comprise heating the compositestructure at a first, lower temperature followed by heating thecomposite structure at a second, higher temperature. For example, invarious embodiments, the composite structure may undergo a first heattreatment at a first temperature of about 250° F. (121° C.) to about350° F. (177° C.) followed by a second heat treatment at a secondtemperature of about 1600° F. (871° C.) to about 1700° F. (927° C.). Invarious embodiments, the first temperature may be about 300° F. (149°C.), and the second temperature may about 1650° F. (899° C.). As used inthis context only, the term “about” means plus or minus 25° F. (4° C.).In various embodiment, the second temperature is selected to be belowthe working point of the glass compound, for example, below the workingpoint of the borosilicate glass in the boron-silicon-glass compositeslurry.

Further, step 230 may be performed in an inert environment, such asunder a blanket of inert or less reactive gas (e.g., nitrogen (N₂),argon, other noble gases, and the like). The composite structure may beheated prior to application of the boron-silicon-glass composite slurryto aid in the penetration of the boron-silicon-glass composite slurry.The temperature rise may be controlled at a rate that removes waterwithout boiling and provides temperature uniformity throughout thecomposite structure.

Step 230 may, for example, comprise heating the composite structure atthe first temperature for a period between about 5 minutes to 8 hours,about 10 minutes to 2 hours, about 10 minutes, and/or about 1 hour,wherein the term “about” in this context only means plus or minus tenpercent of the associated value. Step 230 may, for example, compriseheating the composite structure at the second temperature for a periodbetween about 5 minutes to 8 hours, about 0.5 hours to 4 hours, about1.5 hours to about 2.5 hours, and/or about 2 hours, wherein the term“about” in this context only means plus or minus ten percent of theassociated value. In various embodiments, step 230 may be the finalstep, as method 200 generally does not include a sealing layer step, asmay be associated with other oxidation protection systems, for example,oxidation protection systems comprising phosphate glass layers. In thisregard, the boron-silicon-glass layer may form an exterior surface ofthe oxidation protection system. Stated differently, theboron-silicon-glass layer extends from the surface of the compositesubstrate to the exterior surface of oxidation protection system.

With additional reference to FIG. 1, wear surfaces, such as wear surface33, of brake disks may reach extremely high temperatures duringoperation (temperatures in excess of 1093° C. (2000° F.)). At suchextreme temperatures of wear surfaces, the oxidation protection systemson non-wear surfaces adjacent to the wear surface (e.g., non-wearsurface 45 adjacent to wear surface 33) may experience heating. As usedherein, a “wear” surface refers to a surface of a friction disk thatphysically contacts an adjacent friction disk surface. As used herein, a“non-wear” surface refers to a surface of a friction disk that does notphysically contact the surface of an adjacent friction disk. Oxidationprotection systems comprising a phosphate glass disposed on non-wearsurface 45 (such as that represented by data set 300 in FIG. 3,discussed herein) may increase temperature to a point at which theviscosity decreases and causes beading and/or migration of the oxidationprotection system layers proximate edges 41, 43 away from edges 41, 43and the adjacent wear surfaces (e.g., wear surface 33). Edges 41, 43 andwear surface 33, as labeled in FIG. 1A, simply reference exemplary edgesand an exemplary wear surface, respectively, on a brake disk, but edgessimilar to edges 41, 43 and wear surfaces similar to wear surface 33 maybe present on any brake disks (e.g., rotors 32, stators 34, pressureplate 36, end plate 38, or the like).). Thus, composite material onnon-wear surface 45 proximate edges 41, 43 may be vulnerable tooxidation because of such migration. Additionally, the extremetemperatures during the operation of brake disks may cause cracks withinan oxidation protection system, allowing oxygen to reach the material ofthe composite structure, causing oxidation and material loss.

During operation, at elevated temperatures (e.g., around 1700° F. (927°C.) or 1800° F. (982° C.)), oxygen may diffuse through, or travelthrough cracks in the boron-silicon-glass layer of the oxidationprotection system and oxidize the boron compound in theboron-silicon-glass layer into boron trioxide (B₂O₃). In variousembodiments, in response to temperatures elevating to a sufficient level(e.g., around 1700° F. (927° C.) or 1800° F. (982° C.)), the boroncompound in the boron-silicon-glass layer may be oxidized into borontrioxide, and the silicon compound may react (e.g., oxidize) to formsilica. The silica may react with the boron trioxide to formborosilicate glass. During operating at lower temperatures (e.g., below1700° F. (927° C.)), the silicon compound in the boron-silicon-glasslayer may comprise silicon carbide, as the silicon-compound may notoxidize under such conditions to form the silica to react with the borontrioxide.

The borosilicate glass may be formed in the cracks of theboron-silicon-glass layer. Therefore, the oxidation protection systemsdescribed herein have self-healing properties to protect against cracksformed in the layers of the oxidation protection systems, preventing ormitigating against oxygen penetration and the resulting oxidation andloss of material. Additionally, borosilicate glass has a high viscosity(a working point of about 1160° C. (2120° F.), wherein “about” meansplus or minus 100° C. (212° F.), and the working point is the point atwhich a glass is sufficiently soft for the shaping of the glass). Thus,the high temperatures experienced by edges 41, 43 in their proximity towear surface 33 may cause minimal, if any, migration of the oxidationprotection systems described herein.

Exposure of the boron-silicon-glass layer to moisture after exposure toincreased temperatures (e.g., temperatures greater than 1700° F. (927°C.)), may lead to increased boric acid formation. Boron-silicon-glasslayers having larger average particle size boron carbide and smalleraverage particle size silicon carbide may reduce formation of boric acidby limiting free boron trioxide by reducing the reaction surface area ofboron carbide and increasing the reaction surface area of siliconcarbide. In various embodiments, the boron-silicon-glass layer may beformed by forming and applying a boron-silicon-glass composite slurry(steps 210 and 220) comprised of about 22.581% by weight boron carbidehaving an average particle size of about 9.3 μm (0.0004 inch), about29.032% by weight silicon carbide having an average particle size ofabout 1 μm (4×10⁻⁵ inch), about 12.903% by weight borosilicate glasshaving an average particle size of about 12.3 μm (0.0005 inch), andabout 35.484% by weight water; the borosilicate glass being comprised,in weight percentage of the borosilicate glass, of about 13% B₂O₃, about61% SiO₂, about 2% Al₂O₃, and about 4% Na₂O. As used in this contextonly, the term “about” plus or minus ten percent of the associatedvalue.

With the oxidation protection systems and methods disclosed hereincomprising a boron-silicon-glass layer disposed on a compositestructure, the boron compound, the silicon compound, and/or the glasscompound in the boron-silicon-glass layer may prevent, or decrease therisk of, the oxidation protection system migrating from edges ofnon-wear surfaces adjacent to wear surfaces of composite structures, andadditionally give the oxidation protection system self-healingproperties to mitigate against oxidation caused by cracks in theoxidation protection system layers. The described boron-silicon-glasslayer tends to provide a barrier layer on the non-wear surface at lowtemperatures (e.g., temperatures below 1700° F. (927° C.)) and formborosilicate glass at high temperatures (temperature above 1700° F.(927° C.)). Thus, the oxidation systems and methods described herein mayprevent, or decrease the risk of, the oxidation protection system losingmaterial resulting from such oxidation.

TABLE 1 illustrates two slurries comprising an oxidation protectioncomposition (e.g., a boron-silicon-glass composite slurry describedherein) prepared in accordance with various embodiments. Each numericalvalue in TABLE 1 is the number of grams of the particular substanceadded to the slurry.

TABLE 1 Example >> A B Boron Carbide (B₄C) 0.7 μm 10.50 10.50 BoronCarbide (B₄C) 9.3 μm 24.50 24.50 H₂O 55.0 55.0 BSG 7740 10.0 10.0Silicon Carbine (SiC) 17.0 μm 45.0 31.50 Silicon Carbine (SiC) 1.0 μm0.0 13.5 Total (g) 145.0 145.0 Weight % solids 62.07% 62.07%

As illustrated in TABLE 1, oxidation protection systems comprising aboron-silicon-glass composite slurry, which included a composition ofboron carbide, borosilicate glass comprising 13% B₂O₃, 61% SiO₂, 2%Al₂O₃, and 4% Na₂O weight percent of the borosilicate glass, and siliconcarbide were prepared.

TABLE 2 illustrates two slurries, which may be applied separately fromone another to form an oxidation protection system having a base layercomprising a phosphate glass. Each numerical value in TABLE 2 is thenumber of grams of the particular substance added to the slurry.

TABLE 2 Example >> D E h-Boron nitride powder 0 8.25 Graphenenanoplatelets 0 0.15 H₂O 52.40 60.00 Surfynol 465 surfactant 0 0.20Ammonium dihydrogen phosphate (ADHP) 11.33 0 Glass frit 34.00 26.5Aluminum orthophosphate (o-AlPO₄) 2.270 0 Acid Aluminum Phosphate (AALP)1:2.5 0 5.0

As illustrated in TABLE 2, oxidation protection system slurriescomprising a base layer slurry composition, which included phosphateglass composition glass frit and various additives such as h-boronnitride, graphene nanoplatelets, acid aluminum phosphate, aluminumorthophosphate, a surfactant, a flow modifier such as, for example,polyvinyl alcohol, polyacrylate or similar polymer, ammonium dihydrogenphosphate, and/or ammonium hydroxide, in a carrier fluid (i.e., water)were prepared. Slurry E is applied prior to slurry D and forms aphosphate glass base layer. Slurry D may be a phosphate glass, whichserves as a sealing layer. After applying slurry D and E, the compositeis heated to 1650° F. (899° C.) for two hours.

TABLE 3 Example >> F G Monoaluminum Phosphate (MALP) sol'n 60 75Phosphoric Acid 20 25 H₂O 19 0 BYK-346 surfactant 1 0 Silicon Carbide 00 Boron Carbide 0 0

TABLE 3 illustrates two slurries, which may be applied separately fromone another on top of CVD formed boron carbide layer two to five micronsthick and a CVD formed silicon carbide layer 90 to 120 microns thick toform an oxidation protection system having a phosphate glass sealinglayer. Each numerical value in TABLE 3 is the number of grams of theparticular substance added to the slurry. Slurry F comprisesmonoaluminum phosphate and phosphoric acid, along with water and asurfactant (to form a first sealing layer F after heating), and slurry Gis an exemplary second sealing slurry comprising monoaluminum phosphateand phosphoric acid (to form a second sealing layer G after heating).The MALP solution used was 50% by weight monoaluminum phosphate and 50%by weight water.

TABLE 1, TABLE 2, TABLE 3 and FIG. 3 may allow evaluation of anoxidation protection system comprising a boron-silicon-glass layerformed using a boron-silicon-glass composite slurry, as described hereinversus an oxidation protection system including phosphate glass baselayer and formed using multiply distinct slurries and versus anoxidation protection system including a phosphate sealing layer andformed using CVD. Percent weight loss is shown on they axis and exposuretime is shown on the x axis of the graph depicted in FIG. 3.

For preparing the oxidation protection system comprising slurry A, theperformance of which is reflected by data set 304, slurry A was appliedto a 50-gram first carbon-carbon composite structure coupon and cured ininert atmosphere under heat at 300° F. (149° C.) for 1 hours and underheat at 1650° F. (899° C.) for two hours.

For preparing the oxidation protection system comprising slurry B, theperformance of which is reflected by data set 306, slurry B was appliedto a 50-gram first carbon-carbon composite structure coupon and cured ininert atmosphere under heat at 300° F. (149° C.) for 1 hours and underheat at 1650° F. (899° C.) for two hours.

For preparing the oxidation protection system having a phosphate glassbase layer, the performance of which is reflected by data set 300,slurry E was applied to a 50-gram first carbon-carbon compositestructure coupon and cured in inert atmosphere under heat at 899° C.(1650° F.) to form a base layer. After cooling, slurry D was appliedatop the cured base layer and the coupons were fired again in an inertatmosphere.

For preparing the oxidation protection system comprising CVD formedlayers, the performance of which is reflected by data set 302, a boroncarbide layer two to five microns thick was formed on a 50-gram secondcarbon-carbon composite structure coupon using CVD, a silicon carbidelayer 90 to 120 microns thick was formed on the boron carbide layerusing CVD. Slurry F was applied atop the silicon layer and the compositewas baked for one hour at 300° F. (149° C.). Slurry G was then appliedand the composite was baked for one hour at 300° F.). (149°. Thecomposite was then baked for two hours at 718° C. (1324° F.) to form asealing layer F/G.

After cooling, the coupons were subjected to isothermal oxidationtesting at 1700° F. (927° C.) over a period of hours while monitoringmass loss

As can be seen in FIG. 3, the oxidation protection systems formed usinga boron-silicon-glass composite slurry, reflected by data sets 304 and306 resulted in approximately 200 times less weight loss of thecomposite structure, as compared to the oxidation protection systemrepresented by data set 300 demonstrated weight loss of the compositestructure that was similar to, or less than as demonstrated by data set306, that of the CVD applied oxidation protection system, represented bydata set 302. Table 3 indicates that the oxidation protection systemsformed using a boron-silicon-glass composite slurry may be moreeffective at oxidation protection than oxidation protection systemscomprising phosphate glass base layer and formed using multiple slurriesand almost equally and/or more effective than oxidation protectionsystems having similar layers formed using CVD.

With reference to FIG. 4, a method 400 for forming an oxidationprotection system on composite structure is illustrated. In accordancewith various embodiments, method 400 may, for example, comprise applyingan oxidation inhibiting composition to non-wearing surfaces ofcarbon-carbon composite brake components, such as non-wear surfaces 45and/or lugs 54. Non-wear surface 45, as labeled in FIG. 1A, simplyreferences an exemplary non-wear surface on a brake disk, but non-wearsurfaces similar to non-wear surface 45 may be present on any brakedisks (e.g., rotors 32, stators 34, pressure plate 36, end plate 38, orthe like). In various embodiments, method 200 may be used on the backface of pressure plate 36 and/or end plate 38, an inner diameter (ID)surface of stators 34 including slots 56, as well as outer diameter (OD)surfaces of rotors 32 including lugs 54. The oxidation inhibitingcomposition of method 400 may be applied to preselected regions of acarbon-carbon composite structure that may be otherwise susceptible tooxidation. For example, aircraft brake disks may have the oxidationinhibiting composition applied on or proximate stator slots 56, rotorlugs 54, and/or non-wear surface 45. Method 400 may be performed ondensified carbon-carbon composites. In this regard, method 400 may beperformed after carbonization and densification of the carbon-carboncomposite.

In various embodiments, method 400 may comprise forming a boron slurry(step 402) by combining a boron compound and a glass compound with acarrier fluid (such as, for example, water). In various embodiments, theboron compound may comprise at least one boron-comprising refractorymaterial (e.g., ceramic materials). In various embodiments, the boroncompound may comprise titanium diboride, boron nitride, boron carbide,zirconium boride, silicon hexaboride, and/or elemental boron. In variousembodiments, the glass compound may comprise a borosilicate glass.

Not to be bound by theory, it is presumed that boron components maybecome oxidized during service at high temperatures (e.g., temperaturesgreater than 1300° F. (704° C.)). The boron trioxide may then come intocontact with oxidized silicon components to form a borosilicate in situ,providing a method of self-healing. For a boron-silicon oxidationprotection system, the probability of boron trioxide reacting withoxidized silicon compounds is kinetically controlled and is influencedby the amount of components, surface area, aspect ratio, etc. Borontrioxide is also volatile, especially when hydrated to form boric acid,and may be lost during extended service time. Method 400 increases theprobability of self-healing borosilicate formation by creating a layerof silicon compound by which boron trioxide must transport through priorto volatilization, reducing the dependence on aspect ratio and totalamount of components in the slurry.

The weight percentage of the boron compound within the boron slurry maybe any suitable weight percentage for the desired application. Invarious embodiments, the boron-silicon-glass composite slurry maycomprise from 5% to 50% by weight boron compound, from 15% to 40% byweight boron compound, from 30% to 40% by weight boron compound, and/orabout 35% by weight boron compound. As used in this context only, theterm “about” means plus or minus 1 weight percent.

In various embodiments, the boron compound may comprise boron powder(e.g., boron carbide powder). The boron compound comprises particleshaving an average particle size may be between 100 nanometers (nm) and100 micrometers (μm) (between 3.9×10⁻⁶ inch and 0.0039 inch), between500 nm and 100 μm (between 2×10⁻⁵ inch and 0.0039 inch), between 500 nmand 1 μm (between 2×10⁻⁵ inch and 3.9×10⁻⁵ inch), between 1 μm and 50 μm(between 3.9×10⁻⁵ inch and 0.002 inch), between 1 μm and 20 μm (between3.9×10⁻⁵ inch and 0.0008 inch), and/or between 1 μm and 10 μm (between3.9×10⁻⁵ inch and 0.0004 inch), about 0.7 μm (2.8×10⁻⁵ inch), about 9.3μm (0.0004 inch), and/or about 50 μm (0.0020 inches). As used in thiscontext only, the term “about” means plus or minus ten percent of theassociated value.

In various embodiments, the boron compound may comprise particles ofvarying size. In various embodiments, the boron compound comprises afirst group of particles having a first average particle size and asecond group of particles having a second average particle size greaterthan the first average particle size.

In various embodiments, the particles of the first group may have anaverage particle size between 100 nm and 20 μm (between 3.9×10⁻⁶ inchand 0.0008 inch), between 500 nm and 10 μm (between 2×10⁻⁵ inch and0.0004 inch), between 500 nm and 1.0 μm (2×10⁻⁵ inch and 3.9×10⁻⁵ inch),and/or about 0.7 μm (2.8×10⁻⁵ inch). As used in this context only, theterm “about” means plus or minus ten percent of the associated value.

In various embodiments, the particles of the second group may have anaverage particle size between 500 nm and 60 μm (2×10⁻⁵ inch and 0.0008inch), 1 μm and 15 μm (between 2×10⁻⁵ and 0.0006 inch), between 8 μm and10.0 μm (between 0.0003 inch and 0.0008 inch), between 45 μm and 55 μm(between 0.0018 inch and 0.0022 inch), about 50 μm (0.0020 inch), and/orabout 9.3 μm (0.00039 inch). As used in this context only, the term“about” means plus or minus ten percent of the associated value.

In various embodiments, the boron compound particles of the secondaverage particle size form a larger weight percentage of the boronslurry as compared to the weight percentage formed by the boron compoundparticles of the first size. In various embodiments, theboron-silicon-glass composite slurry may comprise from 0% to 20% byweight the boron compound first average particle size, 5% to 15% byweight the boron compound first average particle size, and/or about10.5% by weight the first boron compound average particle size. As usedin this context only, the term “about” means plus or minus 0.5% weightpercent. In various embodiments, the boron slurry may comprise from 5%to 50% by weight the second boron compound average particle size, 10% to30% by weight the second boron compound average particle size, 20% to25% by weight the second boron compound average particle size, and/orabout 24.5% by weight the second boron compound average particle size.As used in this context only, the term “about” means plus or minus 0.5%weight percent.

In accordance with various embodiments, the boron slurry includes aglass compound. In various embodiments, the glass compound is aborosilicate glass composition in the form of a glass frit, powder orother suitable pulverized form and comprising SiO₂, B₂O₃, and/or Al₂O₃.The borosilicate glass may comprise in weight percentage 13% B₂O₃, 61%SiO₂, 2% Al₂O₃, and 4% Na₂O, and may have a CTE of 3.3×10⁻⁶ cm/Cm, aworking point of 2286° F. (1252° C.), and an annealing point of 1040° F.(560° C.). In various embodiments, the glass compound may comprise alithia potash borosilicate composition in the form of a glass frit,powder or other suitable pulverized form and having a CTE of 3.0×10⁻⁶cm/Cm, a working point of 1954° F. (1068° C.), and an annealingtemperature of 925° F. (496° C.). In various embodiments, the glasscompound may comprise one or more borophosphates, vycor comprised of, inweight percentage. 96% SiO₂ and 4% B₂O₃, quartz, aluminosilicate,boroaluminosilicate, and/or any other suitable glass compound, which maybe in the form of a glass frit, powder, or other suitable pulverizedform.

The weight percentage of the glass compound within the boron slurry maybe any suitable weight percentage for the desired application. Invarious embodiments, the boron-slurry may comprise from 1% to 35% byweight glass compound, from 5% to 15% by weight glass compound, and/orabout 10% by weight glass compound. As used in this context only, theterm “about” means plus or minus 0.5 weight percent.

In various embodiments, the glass compound comprises particles, theaverage particle size may be between 500 nm and 50 μm (between 3.9×10⁻⁶inch and 0.0039 inch), between 5 μm and 25 μm (between 2×10⁻⁵ inch and0.0039 inch), between 10 μm and 15 μm (between 2×10⁻⁵ inch and 3.9×10⁻⁵inch), between 11.5 μm and 13 μm (between 3.9×10⁻⁵ inch and 0.002 inch),and/or between about 12.3 μm (0.0004 inch). As used in this contextonly, the term “about” means plus or minus 0.5 weight percent.

In various embodiments, method 400 further comprises applying the boronslurry to a composite structure (step 404). Applying the boron slurrymay comprise, for example, spraying or brushing the boron slurry to anouter surface of the composite structure. In various embodiments, theapplication of the boron slurry to the composite structure may notcomprise CVD, thus saving the significant monetary expense associatedwith CVD. Any suitable manner of applying the boron-slurry to thecomposite structure is within the scope of the present disclosure,except in various embodiments, CVD. As referenced herein, the compositestructure may refer to a carbon-carbon composite structure. Inaccordance with various embodiments, the boron slurry may be applieddirectly on (i.e., in physical contact with) the surface of thecomposite structure. In this regard, method 400 generally does notinclude a pretreating composition and/or a pretreating step and/orforming a sealing layer prior to applying the boron slurry or siliconslurry, as may be associated with other oxidation protection systems,for example, oxidation protection systems comprising phosphate glasslayers.

In various embodiments, method 400 may further comprise performing afirst low temperature bake (step 406). Step 406 may include heating thecomposite structure at a relatively low temperature (for example, atemperature of about 250° F. (121° C.) to about 350° F. (177° C.),and/or at about 300° F. (149° C.), wherein the term “about” in thiscontext only means plus or minus 25° F. (4° C.)). Step 406 may includeheating the composite structure for about 5 minutes to 8 hours, about 10minutes to 2 hours, and/or about 1 hour, wherein the term “about” inthis context only means plus or minus ten percent of the associatedvalue.

In various embodiments, method 400 may comprise forming a silicon slurry(step 408) by combining a silicon compound and a glass compound with acarrier fluid (such as, for example, water). In various embodiments, thesilicon compound may comprise silicon carbide, a silicide compound,silicon, and/or silicon carbonitride.

The weight percentage of the silicon compound within the silicon slurrymay be any suitable weight percentage for the desired application. Invarious embodiments, the silicon slurry may comprise from 10% to 70% byweight silicon compound, from 20% to 55% by weight silicon compound,from 40% to 50% by weight silicon compound, and/or about 45% by weightsilicon compound. As used in this context only, the term “about” meansplus or minus 2 weight percent.

In various embodiments, the silicon compound may comprise a siliconpowder (e.g., silicon carbide powder). The silicon compound comprisesparticles, the silicon compound average particle size may be between 100nm and 50 μm (between 3.9×10⁻⁶ inch and 0.0039 inch), between 500 nm and20 μm (between 2×10⁻⁵ inch and 0.0039 inch), between 500 nm and 1.5 μm(between 2×10⁻⁵ inch and 3.9×10⁻⁵ inch), between 15 μm and 20 μm(between 3.9×10⁻⁵ inch and 0.002 inch), about 17 μm (0.0007 inch),and/or about 1.0 μm (4×10⁻⁵ inch). As used in this context only, theterm “about” means plus or minus ten percent of the associated value.

In various embodiments, the silicon compound may comprise siliconcompound particles of varying size. In various embodiments, the siliconcompound comprises a first group of silicon compound particles having afirst average particle size and a second group of silicon compoundparticles having a second average particle size. The first siliconcompound average particle size may be less than the second siliconcompound average particle size.

In various embodiments, the silicon compound particles of the firstgroup have an average particle size between 100 nm and 20 μm (between3.9×10⁻⁶ inch and 0.0008 inch), 500 nm and 10 μm (between 2×10⁻⁵ and0.0004 inch); between 500 nm and 1.5 μm (between 2×10⁻⁵ and 6×10⁻⁵),and/or about 1.0 μm (4×10⁻⁵ inch). In various embodiments, the siliconcompound particles of the second group may have an average particle sizebetween 1 μm and 50 μm (between 4×10⁻⁶ inch and 0.002 inch), between 5μm and 25 μm (between 0.0002 inch and 0.001), between 16 μm and 18 μm(between 0.0006 inch and 0.0007 inch), and/or about 17 μm (0.00067inches). As used in this context only, the term “about” means plus orminus ten percent of the associated value.

In various embodiments, the silicon compound particles of the firstaverage particle size form a larger weight percentage of theboron-silicon-glass composite slurry as compared to the weightpercentage formed by the silicon compound particles of the secondaverage particle size. In various embodiments, the silicon slurry maycomprise from 0% to 30% by weight the second silicon compound averageparticle size, 0% to 20% by weight the second silicon compound averageparticle size, and/or about 13% by weight the second silicon compoundaverage particle size. As used in this context only, the term “about”means plus or minus 1% weight percent. In various embodiments, thesilicon slurry may comprise from 5% to 80% by weight the first siliconcompound average particle size, 20% to 60% by weight the first siliconcompound average particle size, 30% to 50% by weight the first siliconcompound average particle size, about 31.5% by weight the second siliconcompound average particle size, and/or about 45.0% by weight the secondsilicon compound average particle size. As used in this context only,the term “about” means plus or minus 1% weight percent.

In various embodiments, the silicon slurry may include a glass compound.The glass compound may comprise a borosilicate glass composition in theform of a glass frit, powder, or other suitable pulverized form. Invarious embodiments, the borosilicate glass composition may compriseSiO₂, B₂O₃, and/or Al₂O₃. The borosilicate glass may comprise in weightpercentage 13% B₂O₃, 61% SiO₂, 2% Al₂O₃, and 4% Na₂O, and may have a CTEof 3.3×10⁻⁶ cm/Cm, a working point of 2286° F. (1252° C.), and anannealing point of 1040° F. (560° C.). In various embodiments, the glasscompound may comprise a lithia potash borosilicate composition in theform of a glass frit, powder, or other suitable pulverized form andhaving a CTE of 3.0×10⁻⁶ cm/Cm, a working point of 1954° F. (1068° C.),and an annealing temperature of 925° F. (496° C.). In variousembodiments, the glass compound may comprise one or more borophosphates,vycor comprised of, in weight percentage. 96% SiO₂ and 4% B₂O₃, quartz,aluminosilicate, boroaluminosilicate, and/or any other suitable glasscompound, which may be in the form of a glass frit, powder, or othersuitable pulverized form.

The weight percentage of the glass compound within the silicon slurrymay be any suitable weight percentage for the desired application. Invarious embodiments, the silicon slurry may comprise from 1% to 35% byweight glass compound, from 5% to 15% by weight glass compound, and/orabout 10% by weight glass compound. As used in this context only, theterm “about” means plus or minus 0.5 weight percent.

In various embodiments, the glass compound comprises particles, theaverage particle size may be between 500 nm and 50 μm (between 3.9×10⁻⁶inch and 0.0039 inch), between 5 μm and 25 μm (between 2×10⁻⁵ inch and0.0039 inch), between 10 μm and 15 μm (between 2×10⁻⁵ inch and 3.9×10⁻⁵inch), between 11.5 μm and 13 μm (between 3.9×10⁻⁵ inch and 0.002 inch),and/or between about 12.3 μm (0.0004 inch). As used in this contextonly, the term “about” means plus or minus 0.5 weight percent.

In various embodiments, method 400 further comprises applying thesilicon slurry to a composite structure (step 410). The silicon slurrymay be applied over the boron compound of the boron slurry and after thelow temperature bake of step 406. In this regard, in variousembodiments, the only heat treatment between application of the boronslurry (step 404) and the application of the silicon slurry (step 410)may be the first low temperature bake (step 406). Applying the siliconslurry may comprise, for example, spraying or brushing the boron slurryto an outer surface of the composite structure. In various embodiments,the application of the silicon slurry to the composite structure may notcomprise CVD, thus saving the significant monetary expense associatedwith CVD. Any suitable manner of applying the silicon slurry to thecomposite structure is within the scope of the present disclosure,except in various embodiments, CVD. As referenced herein, the compositestructure may refer to a carbon-carbon composite structure.

In various embodiments, method 400 may further comprise performing asecond low temperature bake (step 412). Step 412 may include heating thecomposite structure at a relatively low temperature (for example, atemperature of about 250° F. (121° C.) to about 350° F. (177° C.),and/or at about 300° F. (149° C.), wherein the term “about” in thiscontext only means plus or minus 25° F. (4° C.)). Step 412 may includeheating the composite structure for about 5 minutes to 8 hours, about 10minutes to 2 hours, and/or about 1 hour, wherein the term “about” inthis context only means plus or minus ten percent of the associatedvalue.

In various embodiments, method 400 may further comprise performing ahigh temperature heat treatment to form the boron layer and the siliconlayer on the composite structure (step 414). Step 414 may includeheating the composite structure at a relatively high temperature (forexample, a temperature of about 1500° F. (816° C.) to about 1700° F.(927° C.), and/or at about 1650° F. (899° C.), wherein the term “about”in this context only means plus or minus 25° F. (4° C.)). Step 414 mayinclude heating the composite structure for about 5 minutes to 8 hours,about 30 minutes to 5 hours, and/or about 2 hours, wherein the term“about” in this context only means plus or minus 0.5 hours. Step 414 isperformed after the second low temperature bake (step 412).

During operation, at elevated temperatures (e.g., around 1700° F. (927°C.) or 1800° F. (982° C.)), oxygen may diffuse through, or travelthrough cracks in the silicon layer of the oxidation protection systemand oxidize the boron compound in the boron layer into B₂O₃. In variousembodiments, in response to temperatures elevating to a sufficient level(e.g., around 1700° F. (927° C.) or 1800° F. (982° C.)), the siliconcompound may react (e.g., oxidize) to form silica. The silica may reactwith the boron trioxide to form borosilicate glass.

The borosilicate glass may be formed in the cracks of the silicon layer.Therefore, the oxidation protection systems described herein haveself-healing properties to protect against cracks formed in the layersof the oxidation protection systems, preventing or mitigating againstoxygen penetration and the resulting oxidation and loss of material.Additionally, borosilicate glass has a high viscosity (a working pointof about 1160° C. (2120° F.), wherein “about” means plus or minus 100°C. (212° F.), and the working point is the point at which a glass issufficiently soft for the shaping of the glass). Thus, the hightemperatures experienced by edges 41, 43 in their proximity to wearsurface 33 may cause minimal, if any, migration of the oxidationprotection systems described herein.

Exposure of the boron layer to moisture after exposure to increasedtemperatures (e.g., temperatures greater than 1700° F. (927° C.)), maylead to increased boric acid formation. In accordance with variousembodiments, oxidation protection system formed using method 400 mayinclude a boron layer having only larger average particle size boroncarbide (e.g., particles about 9.0 μm or greater (0.0004 inch)), and asilicon layer having only smaller average particle size silicon carbide(e.g., particles about 1.0 μm or less (4>10⁻⁵ inch). Oxidationprotection systems having increased percentages of larger averageparticle size boron carbide (e.g., particles about 9.0 μm or greater(0.0004 inch)) and smaller average particle size silicon carbide (e.g.,particles about 1.0 μm or less (4×10⁻⁵ inch) may reduce formation ofboric acid by limiting free boron trioxide by reducing the reactionsurface area of boron carbide and increasing the reaction surface areaof silicon carbide.

Benefits and other advantages have been described herein with regard tospecific embodiments. Furthermore, the connecting lines shown in thevarious figures contained herein are intended to represent exemplaryfunctional relationships and/or physical couplings between the variouselements. It should be noted that many alternative or additionalfunctional relationships or physical connections may be present in apractical system. However, the benefits, advantages, solutions toproblems, and any elements that may cause any benefit, advantage, orsolution to occur or become more pronounced are not to be construed ascritical, required, or essential features or elements of the disclosure.The scope of the disclosure is accordingly to be limited by nothingother than the appended claims, in which reference to an element in thesingular is not intended to mean “one and only one” unless explicitly sostated, but rather “one or more.” Moreover, where a phrase similar to“at least one of A, B, or C” is used in the claims, it is intended thatthe phrase be interpreted to mean that A alone may be present in anembodiment, B alone may be present in an embodiment, C alone may bepresent in an embodiment, or that any combination of the elements A, Band C may be present in a single embodiment; for example, A and B, A andC, B and C, or A and B and C.

Systems, methods and apparatus are provided herein. In the detaileddescription herein, references to “one embodiment,” “an embodiment,” “anexample embodiment,” etc., indicate that the embodiment described mayinclude a particular feature, structure, or characteristic, but everyembodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed. After reading the description, it will be apparent to oneskilled in the relevant art(s) how to implement the disclosure inalternative embodiments.

Furthermore, no element, component, or method step in the presentdisclosure is intended to be dedicated to the public regardless ofwhether the element, component, or method step is explicitly recited inthe claims. No claim element herein is intended to invoke 35 U.S.C.112(f), unless the element is expressly recited using the phrase “meansfor.” As used herein, the terms “comprises,” “comprising,” or any othervariation thereof, are intended to cover a non-exclusive inclusion, suchthat a process, method, article, or apparatus that comprises a list ofelements does not include only those elements but may include otherelements not expressly listed or inherent to such process, method,article, or apparatus.

What is claimed is:
 1. An oxidation protection system disposed on anouter surface of a substrate, the oxidation protection system,comprising: a boron-silicon-glass layer disposed on the outer surface,the boron-silicon-glass layer comprising: a boron compound; a siliconcompound; and a glass compound.
 2. The oxidation protection system ofclaim 1, wherein the boron compound comprises a first group of boronparticles each having a first average particle size and a second groupof boron particles each having a second average particle size greaterthan the first average particle size.
 3. The oxidation protection systemof claim 1, wherein an average particle size of a first group of siliconcarbide particles of the silicon compound is less than an averageparticle size of a second group of silicon carbide particles of thesilicon compound.
 4. The oxidation protection system of claim 4, whereinthe boron-silicon-glass layer is formed in direct contact with the outersurface of the substrate.
 5. The oxidation protection system of claim 4,wherein the glass compound comprises a borosilicate glass.
 6. Theoxidation protection system of claim 2, wherein the boron compoundcomprises at least one of titanium diboride, boron nitride, boroncarbide, zirconium boride, silicon hexaboride, or elemental boron, andwherein the silicon compound comprises at least one of silicon carbide,silicon dioxide, a silicide compound, silicon, fumed silica, or siliconcarbonitride.
 7. A method for forming an oxidation protection system ona composite structure, comprising: apply a boron slurry to the compositestructure; performing a first low temperature bake by heating thecomposite structure at a first temperature; applying a silicon slurry tothe composite structure; performing a second low temperature bake byheating the composite structure at a second temperature; and performinga high temperature heat treatment by heating the composite structure ata third temperature, the third temperature being greater than the firsttemperature and the second temperature.
 8. The method of claim 7,wherein the first temperature is about 300° Fahrenheit, the secondtemperature is about 300° Fahrenheit, and the third temperature is about1650° Fahrenheit.